The present invention may be more easily understood in the context of low light imaging arrays such as those used in digital photography to record an image. For the purposes of this discussion, an image will be defined as a two-dimensional array of digital values that represent the amount of light received during an exposure period at each pixel on a two-dimensional surface. It will be assumed that each pixel is a small rectangular area on that surface. In digital photography, the image is recorded by an imaging array in which each pixel includes a photodetector that measures the amount of light that falls on some portion of the pixel area. Image arrays that have a high dynamic range are required for many applications, including scientific research photography. The dynamic range of an imaging array will be defined to be the ratio of the maximum signal for a pixel to the minimum signal that is above the noise.
One class of imaging array in current use is commonly referred to as a CMOS array, as it is produced by a “complementary metal-oxide-semiconductor” or CMOS process. Each pixel in the array includes a photodetector, such as a photodiode or a photogate, and a readout circuit. The readout circuit converts the charge collected by the photodetector to a voltage signal that is transmitted over a bit line that is shared by a number of pixels in the array. The charge conversion circuit is typically a source follower consisting of a single transistor; however, imaging arrays having more complex charge conversion circuits are also known. During readout, each pixel on a given bit line is selectively connected to that bit line and the signal on that bit line is digitized with an analog-to-digital converter that may include an amplifier that sets the effective gain of the analog-to-digital converter.
There are two sources of noise in the digital values recorded from each pixel by the analog-to-digital converter. The first is the sensor noise of the photodiode and the analog readout circuitry, and the second results from the finite steps of the ADC. The sensor noise is the sum of the shot noise from the photodetector, and thermal and 1/f noise from the readout electronics. Shot noise typically dominates the sensor noise for signal levels above 10 photons. The output of the ADC has an uncertainty of one half the voltage difference corresponding to the least significant bit of the digital value. This uncertainty will be referred to as the “quantization noise” in the following discussion.
Recent advances in CMOS imaging technology have resulted in image sensors with extremely low sensor noise levels, corresponding to the signal that would be produced from receiving as few as two photons. To take advantage of an array with such low noise pixels at low light levels, a digitization circuit having low quantization noise and high gain should be utilized. If such a digitization circuit is used to convert signals from pixels having high light intensities, however, the output voltages will be too high for conventional low cost CMOS circuitry to process. If, on the other hand, the amplifier gain in the digitization circuit is set to a low value to maintain the signal within the range of CMOS circuitry when the analog signals from high intensity pixels are processed, the quantization noise will mask the low level signals.